The present invention relates to a thermal ink jet printhead and method of manufacture therefore, and more particularly to an improved thermal ink jet printhead having minimized standoff between two bonded parts by compensating for topographic formations developed in an insulating layer during fabrication.
In existing thermal ink jet printing systems, an ink jet printhead expels ink droplets on demand by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in capillary-filled, parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. U.S. Pat. No. Re. 32,572 to Hawkins et. al. exemplifies such a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing elements (heater plate), and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together (channel plate). A linear array of parallel grooves are also formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing elements on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated manifolds are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
Improvements to such two part thermal ink jet printheads include U.S. Pat. No. 4,638,337 to Torpey et. al. that discloses an improved printhead similar to that of Hawkins et. al., but has each of its heating elements located in a recess (termed heater pit). The recess walls containing the heating elements prevent lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent, a thick film organic structure such as polyimide, Riston.RTM. or Vacrel.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to provide the recesses for the heating elements, so that the bubbles which are formed on the heating elements are laterally constrained, thus enabling an increase in the droplet velocity without the occurrence of vapor blow-out and concomitant air ingestion. U.S. Pat. No. 4,774,530 to Hawkins further refines the two part printhead by disclosing an improvement over the patent to Torpey et. al. Further recesses (termed bypass pits) are patterned in the thick film layer to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess. The heater plates, having the aforementioned improvements of heater pits and bypass pits formed in the thick film organic structure covering the heater plate surface, are aligned with the channel plate, so that each channel groove has a recessed heating element therein.
Thorough bonding between heater and channel plates is paramount to maintaining the efficiency, consistency, and reliability of an ink jet printhead. U.S. Pat. No. 4,678,529 to Drake et. al. discloses a method of bonding ink jet printhead components together by spin coating or spraying a relatively thin, uniform layer of adhesive on a flexible substrate and then manually placing the flexible substrate surface with the adhesive layer against a printhead component surface. A uniform pressure and temperature is applied to ensure adhesive contact with all coplanar surface portions and then the flexible substrate peeled away, leaving a uniformly thin coating on the surfaces to be bonded. A roller or vacuum lamination may be applied to the flexible substrate to insure contact on all of the lands or coplanar surfaces of the printhead part. Unfortunately, this labor intensive method permits adhesive layer thickness variation between a plurality of identical parts, so that ink flow characteristics varies from printhead to printhead. Accordingly, a more mechanized process to place the adhesive coating on the disk with the channel wafer was required to minimize operator involvement and consequent variation in parameters which introduced thickness variations in the amount of adhesive layer transferred to the channel wafers, especially in the thickness variations from wafer-to-wafer. This process is described in U.S. patent application Ser. No. 07/888,220, to Narang et. al., Filed May 26, 1992. The process includes the step of applying a uniform thick layer of adhesive to one surface of a plurality of planar substrates, one substrate at a time, by a method and apparatus which controls both the adhesive layer thickness on each substrate surface and the thickness variations from substrate-to-substrate. As a result, consistent, repeatable, uniformly thick adhesive layers may be applied to each of a plurality of substrates, and the applied layers meet the same tolerance for thickness variation.
Although advances have improved the adhesive layer thickness which bonds the ink jet printhead heater and channel plates, insufficient adhesion between bonded heater and channel plates continues to cause a host of problems affecting channel firing consistency such as different drop sizes between adjacent channels. Since increased adhesive layer thickness is not a practical solution because it tends to spread or wick into the channel, the inter-channel gaps between bonded heater and channel plates must be minimized in order to insure consistent printhead firing characteristics. As taught by the above identified U.S. patents, two wafers are bonded together after alignment for subsequent dicing into individual printheads Each printhead part is formed individually on two separate substrates or wafers, where one contains heating elements and the other ink channels or passageways. The wafer containing the ink channels is silicon, and the channels are formed by an anisotropic etching process. The anisotropic or orientation dependent etching has been shown to be a high yielding process that produces very planar and highly precise channel plates. The other wafer containing the heating elements as well as heater addressing logic is covered by a thick film organic structure in which heater and bypass pits are formed using photolithography. The thick film organic structure used to protect silicon substrates is often formed with polyimide, which is also used as an interconnect material and insulator. Because of its beneficial property of being impervious to water, it is commonly considered a standard material for protecting circuitry on silicon substrates. However, one drawback with the polyimide material is its tendency to form unwanted topographical formations, such as raised edges or lips (1-3 microns high) at any photoimaged edge. When bonding both heater and channel plates together, a standoff between the two plates is caused by the raised edges, which reduces the adhesiveness of the bond between the two plates and which cause the formation of inter-channel gaps.
Polyimide topography, such as raised edges, are undesirable by-products resulting from photoimaged heater and bypass pits on heater plates. The raised edges, are polyimide topographical features that critically interfere with the proximity at which heater and channel plates are bonded together. Raised edges, however, are not the only topographical formation created from photoimaged polyimide. Other topographical formations, such as wall sags or dips, compound the negative effects of raised edges by adding to the standoff between the bonded heater and channel plates. Wall dips are slumps in the polyimide walls between polyimide photoimaged pits. The polyimide sandwiched between the two wafers or plates can form more than 2 microns of topographical variation, which does not allow the bonding adhesive, approximately 2 microns or less thick, to bridge or fill in the formation of inter-channel gaps. These inter-channel gaps can allow crosstalk between channels when drops are being ejected. As the patent '529 to Drake et. al. teaches, care must be taken when applying adhesive in bonding the channel and heater plates so as to insure all fluid surfaces in contact with ink are free of adhesive in order that they are not obstructed during operation. There exists, therefore, a need to improve the adhesion between the bonded heater and channel plates in order to minimize inter-channel gaps by reducing the standoff between the butted plates without increasing the amount of adhesive or epoxy used in bonding them.